The present invention relates to a method for fabricating a thin film transistor, and more particularly to a method capable of improving characteristics at the boundary surface between a gate insulation film and an active layer, such as adhesion power, lattice-breakdown, impurity implantation.
In general, thin film transistors are widely employed as switching devices in liquid crystal display devices or contact image sensors.
Therefore, this film transistors must be operated in high speed. So as to operate thin film transistors in high speed, the characteristics at boundary surface between the gate insulation film and a semiconductor serving as the active layer must be good and the carrier concentration of the semiconductor layer must be also high.
FIGS. 1a to 1e are sectional views showing a method for fabricating a conventional thin film transistor.
In accordance with the illustrated method, a polysilicon layer 2 to be used as an active layer is deposited on the whole surface of an insulation transparent substrate 1 such as glass or quartz, as shown in FIG. 1a.
As shown in FIG. 1b, thereafter, the polysilicon layer 2 is patterned using a photo lithography process and an etch process so that it is remained merely at the active layer.
A gate insulation film 3 and a conductive layer 4 to be used as a gate electrode are then formed on the whole surface of the resulting structure, in this order.
As shown in FIG. 1c, subsequently, a gate electrode region is defined using a photo resist 5 for defining and an unnecessary portion of the conductive layer 4 is removed using the photo resist 5 as an etch mask to form a gate electrode 4a.
Impurity ions are implanted in the polysilicon 2 using the gate electrode 4a as an implantation mask, to form a source region 2a and a drain region 2b in the polysilicon layer 2 corresponding to both sides of the gate electrode 4a.
As shown in FIG. 1d, thereafter, a protection film 6 is deposited on the whole surface of the resulting structure.
As shown in FIG. 1e, subsequently, the protection film 6 and the gate insulation film 3 are selectively removed so that the source region 2a and the drain region 2b are exposed, to form a contact hole.
Thereafter, a source electrode 7 and a drain electrode 7a are formed so that they are connected to the source region 2a and the drain region 2b, respectively.
However, the conventional method for fabricating a thin film transistor has the following problems.
First, since the gate insulation film 3 is formed after the polysilicon layer 2 and the active region are formed using the photo resist 5, several type of impurities are flowed upon patterning the polysilicon layer 2 and the photo resist 5 is remained even after the polysilicon layer 2 is patterned, thereby causing the characteristics at the boundary surface between the gate insulation film 3 and the polysilicon layer 2 which is the active layer to be deteriorated.
Second, since impurity ions are implanted in the polysilicon layer through the gate insulation film 3 using the gate electrode as an ion-implantation mask after the gate electrode 4a, a considerable-high energy is required to implant impurity ions in the polysilicon layer through the gate insulation film 3 which has a thickness of above 1000.ANG., in general. It is also difficult to exactly adjust the quantity of impurity ions which reach to the source region and the drain region.
Accordingly, there is a disadvantage in that it is difficult to obtain a thin film transistor capable of operating in high speed.